iio: pressure: bmp280: Fix BMP580 temperature reading

Fix overflow issue when storing BMP580 temperature reading and
properly preserve sign of 24-bit data.

Signed-off-by: Adam Rizkalla <ajarizzo@gmail.com>
Tested-By: Vasileios Amoiridis <vassilisamir@gmail.com>
Acked-by: Angel Iglesias <ang.iglesiasg@gmail.com>
Link: https://lore.kernel.org/r/Zin2udkXRD0+GrML@adam-asahi.lan
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>